91. Hormonal changes during the estrus cycle

Question 1

Overview

Answer 1

-Review of hormonal events
 • Analysis of GnRH surge
• Analysis of the LH surge
• Review of the luteolysis
• Refractory period of the CL

Question 2

Hormones:

Answer 2

• E2
• P4
• PGF2a
• LH
• FSH

Question 3

Ovulation:

Answer 3

• High level of estrogen • GnRH then LH peak
(Resulting in
ovulation)
• High level of estrogen

Question 4

Corpus
haemorragicum,
luteinization:

Answer 4

• Dropping level of estrogen and FSH
• Due to the previous LH-peak, luteinization is in progress
• PRL levels increasing

Question 5

LH peak (ovulation)

Answer 5

• LH-peak induces the ovulation and luteinization process
• LH-peak in different animals:
  • In rabbit and cow: short (6-8 hours)
  • Inpig: 12-18hours
  • In horse: the longest (3-5 days)
• Practical relevance: single GnRH or LH (or analogues) injection, given in the appropriate time, can induce the ovulation in most of the species but in mares. To induce the ovulation in mares, one needs to use repeated injections or implants
• Ovulation follows the LH peak in most of the animals. In mare, the LH-peak disappears in 1-2 day after the ovulation

Question 6

Explain the drop in FSH in relation to GNRH:

Answer 6

The drop in FSH is in relation with the decrease of GnRH levels
• FSH level drops due to the negative effect of progesterone on the hypothalamus. This process is followed by follicular atresia (further development and stimulation is stopped).

Question 7

Early phase of CL

Answer 7

• Slowly increasing P4 levels
  -In domestic animals, in this
  stage, a new generation of follicular cohort start developing, but increasing P4 stops the process (little elevation in E2 and FSH concentrations)

Question 8

Flourishing corpus

luteum

Answer 8

• High level of P4
• Other hormones are on a
baseline concertation
• In domestic animals, in
this phase, new generation of follicles start developing, but these follicles (due to the high level of P4) soon undergo the atresia (no visible hormonal changes in the blood plasma)

Question 9

Luteal phase is :

Answer 9

Growing (early) phase of CL and flourishing CL.

Question 10

What is special in the dogs in growing (early) phase of CL?

Answer 10

P4 levels start elevating before the ovulation (~24 hours): in bitch, the exact timing of ovulation can be determinded by checking the plasma P4 concentration.

Question 11

Flourishing CL:

Answer 11

Plasma E2 is very low (pg/ml) + due to the negative effect of high P4 levels on the CNS,
females are not showing the signs of the estrus
All follicles stops in their development and soon become atretic. No follicular development and ovulation.

Question 12

Luteolysis, early

follicular development

Answer 12

If the animal is not pregnant, PGF2a is produced by the uterus causing the lysis of the CL
• Due to the decreasing P4 levels, FSH concentration start increasing

Question 13

PGF2α release:

Answer 13

• From the endometrium directly to the ovary (Endometrial vein → ovarian artery; Ruminants)
• Through the systemic circulation (Horse)

Question 14

Follicular development and selection

Answer 14

• Increasing FSH levels stimulate the follicular development
• Inhibin is produced by the fastest developing follicle (dominant follicle)
• Inhibin blocks the other developing follicles (on will ovulate)
• In some species, inhibin cause a sharp drop in plasma FSH levels (negative feedback)
• Due to the constantly high level of E2 and FSH, GnRH peak (followed by the LH peak) develops and consequently next ovulation happens

Question 15

Follicular phase

Answer 15

• A new group of follicles start to develop following the P4 drop in blood plasma (luteolysis)
• High E2 level and the lack of P4 inhibitory effects lead to elicit the estrus behavior.

Question 16

Secretion of gonadotropin- releasing hormone (GnRH)

Answer 16

GnRH frequency is influenced by:

• Estrous phase (hormones)
• Season (melatonin)
• Energy balance

Question 17

The relevance of preovulatory estrogen peak:

Answer 17

-According to the classical explanation, rising pre-ovulatory estrogen levels directly prepare the GnRH and LH peaks and therefore the ovulation
• Negative – positive feedback change of the hypothalamic GnRH cells

Question 18

What triggers ovulation?

Answer 18

GnRH in the hypothalamus stimulates the release of LH from the anterior pituitary (LH peak). This increase in LH production only lasts for 24 to 48 hours. This “LH surge” triggers ovulation, thereby not only releasing the egg from the follicle, but also initiating the conversion of the residual follicle into a corpus luteum that, in
turn, produces progesterone to prepare the endometrium for
a possible implantation.

Question 19

LH peak in different species:

Answer 19

Duration of LH peak (ovulation after LH peak)
RU: 6-8h(24h)
Swine:24h(40h)
Dog:.24h(24-48h)
Horse: 3-5 days(2days)

Question 20

Luteolysis (end of luteal phase)

Answer 20

PGF2α production in the non-pregnant uterus leads to the lysis of the CL, and consequently to a sharp drop in plasma P4
-In normal cycle, if the animal is not pregnant, endometrium releases PGF2α on day 16-17 (sheep 13-14) after ovulation.
-Oxytocin increases PGF2α production
-Apoptosis of the luteal cells within hours (sharp decrease in
progesterone level)
-PGF2α
• From the endometrium directly to the ovary (Endometrial vein → ovarian artery; Ruminants)
• Through the systemic circulation (Horse)

Question 21

Regulation of seasonality

Answer 21

The most important stimulator in seasonality is the length of diurnal periods, that determines the „rushing into cycle” process.
- Major role: Melatonin concentrations

During fall:
- the length of dark period>9.5 hours, the„ rushing into cycle” starts for those, which are sensitive for short day light; like the SHEEP. (In others, like MARE, the cycle is inhibited.)

-During spring:
• the length of dark period <9.5 hours, cycle starts for
those, which are sensitive for long day light; like MARE.
The cycle of the sheep is inhibited.